Coleopterans are a significant group of agricultural pests that cause extensive damage to crops each year. Examples of coleopteran pests include corn rootworm and alfalfa weevils. Additional notable examples include Colorado potato beetle, boll weevil, and Japanese beetle.
Insecticidal crystal proteins from some strains of Bacillus thuringiensis (B.t.) are well-known in the art. See, e.g., Höfte et al., Microbial Reviews, Vol. 53, No. 2, pp. 242-255 (1989). These proteins are typically produced by the bacteria as approximately 130 kDa protoxins that are then cleaved by proteases in the insect midgut, after ingestion by the insect, to yield a roughly 60 kDa core toxin. These proteins are known as crystal proteins because distinct crystalline inclusions can be observed with spores in some, strains of B.t. These crystalline inclusions are often composed of several distinct proteins.
A new insecticidal protein system was discovered in Bacillus thuringiensis as disclosed in WO 97/40162. This system comprises two proteins—one of approximately 15 kDa and the other of about 45 kDa. See also U.S. Pat. Nos. 6,083,499 and 6,127,180. These proteins have now been assigned to their own classes, and accordingly received the Cry designations of Cry34 and Cry35, respectively. See Crickmore et al. website (biols.susx.ac.uk/home/Neil_Crickmore/Bt/). Many other related proteins of this type of system have now been disclosed. See e.g. U.S. Pat. No. 6,372,480; WO 01/14417; and WO 00/66742. Plant-optimized genes that encode such proteins, wherein the genes are engineered to use codons for optimized expression in plants, have also been disclosed. See e.g. U.S. Pat. No. 6,218,188.
Details of the three-dimensional structure of these proteins have not, heretofore, been disclosed. With information regarding the three-dimensional structures of these proteins, it would be possible to rationally design modifications to the natural, bacterial proteins to improve various desirable characteristics of these proteins. Having and analyzing the 3D structure of a protein can be highly advantageous for focusing or restricting directed evolution and improvement programs.
However, obtaining purified crystals of B.t. insect toxins has been a difficult process (although some examples do exist; see e.g. WO 98/23641 and WO 99/31248). It has been difficult to obtain purified crystals of adequate quality. For example, there has been a tendency for these proteins to form aggregates that are not suitable for refinement of the structure to high resolution. In addition, B.t. has been an inferior protein producer for the level and quality of protein required for X-ray crystallography and related biochemical purposes. Frequent protease contamination has also been an associated obstacle. Still further, native B.t. strains typically produce crystals having a mixture of proteins; thus, there have been some issues with isolating and purifying single protein types from such mixtures (to the degree required for sophisticated analysis).